This invention relates to a heat-resistant plastic semiconductor device, in particular to a heat-resistant plastic semiconductor device using a flat package, possessing high heat and humidity resistance, and intended for high-density mounting.
As electronic devices are made to perform increasingly many functions and to process increasingly large quantities of information at increasingly high speeds, so-called flat ICs (integrated circuits) using flat plastic packages (FPPs) which are advantageous for high-density surface mounting, automated mounting, etc. on printed circuit boards have been widely adopted for large-scale integrated circuits and other semiconductor devices.
FIG. 3 shows a flat IC, and FIG. 4 shows a cross section through the line B--B. FIG. 4a shows the case in which a junction coating resin (JCR) is not used, while FIG. 4b shows the case in which a JCR is used. In the case of FIG. 3 and FIG. 4a, first the IC chip 23 is die-bonded on an island 22 supported by island supports, then the external leads 21 and the IC chip 23 are wire-bonded with gold wire 24, and the structure is sealed or encapsulated by a mold resin 25. In the case of FIG. 4b, after the bonding of the gold wires 24, the surface of the IC chip 23 is coated with JCR 26 to improve the resistance to humidity.
As shown in FIG. 5, one conventional method of mounting a flat IC 29 having the above structure on a printed circuit board 27 is a partial-heating soldering method, in which only the leads are heated by a soldering iron 30. Recently another method has often been used for items intended for mass production, in which the entire package is heated. Specifically, as shown in FIG. 6a, solder paste is applied to the printed circuit board 27, the flat IC 29 is placed on the printed circuit board 27, then the heating is performed by such means as hot air or near- or far-infrared radiation. Another method, shown in FIG. 6b, is to apply an adhesive 31 to the printed circuit board 27, followed by placing the flat IC 29 on the adhesive 31, thereby attaching it to the printed circuit board 27, and then dipping the assembly in a solder bath.
When the above soldering method of heating the entire package is employed and the pckage is exposed to temperatures in excess of 200.degree. C., moisture that has penetrated the package while the device was left for about a week or more in ambient conditions of 23.degree. C. and 60% RH (relative humidity) vaporizes, generating great stress acting from the inside of the package toward the outside. In addition, the strength of the mold resin is reduced to nearly 0 kg/cm.sup.2 to 0.1 kg/cm.sup.2 for example. Since the IC chip is hermetically sealed, the vapor cannot easily escape to the outside. The result is that the flat IC, particularly one having a thin mold, swells up like a balloon, eventually causing cracks in the upper or lower surface of the mold resin 25 which seriously degrade the visual appearance of the device as shown at A and B in FIG. 7a showing a cross-sectional view of a flat IC 29 with such a visual defect, FIG. 7b showing a top view and FIG. 7c showing a bottom view.
If such a flat IC 29 with a crack is used the gold wire may break at the location of the crack. Moreover, moisture can easily enter at the crack, penetrate to the IC chip 23, and corrode the aluminum interconnections on its surface, eventually causing open circuits that prevent the IC from performing its function.